'89 100hp L4 losing prime

[sawblade723]

Trying to help a friend with a 1989 Merc 100 L4. When the motor is switched off all the gas flows from the hose back into the tank in only a minute or two. He has to pump the bubble every time he wants to start the motor. He also has to use the warmup lever every time to get enough gas to start. Once it starts it idles somewhat rough with a cough every so often, but it smoothes out when reved up. We rebuilt the fuel pump and accelerator pump, installed new hose and bubble, and tightened all hose clamps but it still looses prime. I'm thinking a major air leak somewhere, could it be in the carbs or should I look somewhere else?

 

[Wingedwheel]

Re: '89 100hp L4 losing prime

Did you replace the bulb?

 

[sawblade723]

Re: '89 100hp L4 losing prime

Yes we replaced the hose and bulb. When we pump the bulb it gets tight but not rock hard, you can still hear a small amount of fuel moving thru the lines but we have not found a leak. When you stop pumping the fuel starts to flow back to the tank, you can see it emptying from the inline fuel filter.

 

[Wingedwheel]

Re: '89 100hp L4 losing prime

That shouldn't happen. Maybe your new bulb is defective.

 

[Fuzzytbay]

Re: '89 100hp L4 losing prime

Built in tank> is the tank venting properly?
No vent or poor vent equals a negative pressure, which is a vacum,it could be enough to siphon the fuel back into the tank, at idle or off, yet still be over come while running and draw enough air to allow the fuel pumps to feed the carbs and run, but maybe roughly.